The Cassini Composite Infrared Spectrometer (CIRS) obtained
thermal spectra of the rings during the approach to Saturn.
CIRS is a Fourier-transform spectrometer that measures
radiation in the thermal infrared from 7 microns to 1
millimeter (1400 to 10 cm-1). For 7.5 hours on June 22,
2004, CIRS obtained a sit-and-stare observation of the lit
afternoon ansa of the B ring (centered at 105568.0 km) using
its Focal Plane 1 (FP1) detector with a spectral resolution
of 1.0 cm-1. At this time, the spacecraft was at a ring
range of ~118 Rs, resulting in a 27800 km diameter footprint
filling the entire B-ring ansa, with a phase angle of ~67
degrees.

The resulting FP1 spectrum, covering 10 cm-1 to 600
cm-1, appears relatively featureless and is
characteristic of a Planck curve peaking near 180 cm-1
(corresponding to an effective temperature of 89 K) scaled
by a function of ring optical depth, ring inclination, and
particle phase function. One exception is the presence of a
roll-off in the intensity shortward of 50-60 cm-1
relative to the 89K blackbody curve. This is thought to be
indicative of material properties of the ring particles. We
explore the implications of this characteristic in
conjunction with the absorption and reflective properties of
several ices, e.g. water, ammonia, and carbon dioxide, along
with those of silicates and carbonaceous materials. In this
analysis, we will examine lower limits on particle sizes
observable at these wavelengths. In addition, we will assess
the degree to which Saturnís own thermal emission appears in
the FP1 spectrum.